The invention relates to a splitting method for division of a solid-state starting material into at least two solid-state pieces, and to the use of a material in such a splitting method.
In microelectronics and photovoltaics in particular, wafers, i.e. thin slices or plates, of materials such as silicon, germanium or sapphire are used. At present, these are typically obtained from a solid-state material in the form of a column or block, which is also referred to as an ingot.
Pieces in the form of cylinders or wafers are produced from such ingots, for example, by means of a sawing or breakup method. These pieces may already constitute a wafer, or the pieces obtained are divided further until they have the desired thickness of a wafer to be produced.
In the sawing or breakup method, wire saws or diamond wire saws are usually used, which results in loss of a portion of up to 50% of the original solid-state material in the form of turnings as “kerf loss”, which is especially disadvantageous in the case of costly starting materials.
Moreover, the sawing operation frequently causes damages on the wafer surface, which have to be remedied by means of additional process steps for surface treatment, for example lapping or polishing method steps.
For avoidance of the disadvantages mentioned, for example, DE 10 2012 001 620 A1 discloses a method in which, for production of wafers, a polymer film is applied to the solid-state material by means of adhesive. After the adhesive has cured, the solid-state material is subjected to thermal stress together with the polymer film. By virtue of different thermal properties of the solid-state material and polymer, the solid-state material breaks into two thinner pieces. The polymer film still adheres to one side of one of the two pieces and has to be removed from the surface in a subsequent step.
The method described, referred to hereinafter as “splitting method”, can also be utilized for division of a thick wafer into two thin wafers, by applying polymer films to the two opposite sides of the thick wafer and splitting it into two thin wafers by means of a corresponding thermal treatment.
The effectiveness of such a method is especially dependent on the selection of a polymer having a suitable glass transition temperature (Tg), the thermal conductivity of the polymer and the mechanical properties thereof, such as brittleness, tensile strength and elasticity.
In addition, DE 10 2012 001 620 A1 describes the use of an additional sacrificial layer between the solid-state material and polymer film, which serves to improve the removal of the polymer film after the detachment step, by breaking down or detaching the sacrificial layer, for example by chemical means by addition of suitable reactants.
However, a disadvantage of this method is the long period of time, which may be up to several hours, which passes before the polymer layer is completely removed. This greatly restricts industrial utilization.
For acceleration of the process of polymer removal, it is possible, by means of an appropriate pretreatment, to introduce additional driving forces in the form of suitable tensile stresses that are effective even at room temperature. These lead to an increase in the area of attack for the reactants or the solvent and promote the breakdown or the detachment and dissolution. However, the additional stresses introduced can also result in damage to the split solid-state material after the removal of the polymer, i.e., for example, the wafer, in that it breaks, for example. Associated with this is a deterioration in the overall yield, which reduces the cost advantage of the splitting method.
It is an object of the invention to specify a means of increasing the overall yield, i.e. the efficiency in relation to the raw materials used and the other resources such as energy and labor, in a splitting method.
More particularly, the profile of the polymer removal against time after a splitting method is to be affected in a specific manner.
Preferably, the polymer is to be separable from the divided starting material in a rapid and residue-free manner and without damage thereto.
Advantageously, the polymer should be reusable.
Moreover, process stability is to be increased through minimization of the number of method steps.
Studies showed that the removal of the polymer layer by breakdown, detachment or dissolution is a diffusion-controlled reaction of the reactants/solvents involved. As time advances, the inward and outward transport of the reactants in the gap which forms between the solid-state material and polymer becomes ever more difficult and very significantly slower. Thus, the inward and outward transport of the reaction products and reactants/the solvent and of the dissolved constituents is the diffusion-controlled rate-limiting step.